1. Field of the Invention
This invention relates to a vehicle suspension system, and more particularly to a vehicle suspension system in which suspension arms are connected to the vehicle body by way of rubber bushings.
2. Description of the Prior Art
There has been known a vehicle suspension system which includes a suspension arm whose outer end is connected to a wheel and whose inner end is connected to the vehicle body by way of a rubber bushing so that the suspension arm can vertically rotate relative to the vehicle body. In such a suspension system, it is required that the stiffness of the rubber bushing in the longitudinal direction of thereof, i.e., in the longitudinal direction of the vehicle body, is small in order to enhance the damping capacity against longitudinal vibrations and impacts transmitted to the suspension arm from the road surface during running, thereby improving the driving comfort, and that the stiffness of the rubber bushing in the transverse direction thereof, i.e., the transverse direction of the vehicle body, is large in order to increase the lateral stiffness of the suspension system and improve the driving stability.
In order to meet the requirement, there have been proposed various suspension systems. For example, in the suspension system disclosed in Japanese Utility Model Publication No. 62-11203, tubular portions are formed on the inner end portion of the suspension arm respectively at front and rear portions of the inner end portion and a pair of rubber bushings are respectively mounted on the tubular portions, each of the rubber bushings being a slide bushing having outer and inner tubular members which are slidable relative to each other in the longitudinal direction of the bushing, and at the same time, a hard rubber member is interposed between the outer and inner tubular members of each of the rubber bushings so that the stiffness of the rubber bushing in the transverse direction of the vehicle body is increased, and a soft rubber member which abuts against an end face of the tubular portion on the inner end portion of the suspension arm (e.g., the rear end face) is provided on each of the rubber bushings so that the stiffness of the rubber bushing in the longitudinal direction of the vehicle body reduced.
However, this arrangement is disadvantageous in that since it is actually very difficult to set the hardness of the soft rubber member as desired, the urging force of the soft rubber memeber which acts on the tubular portion on the inner end portion of the suspension arm upon axial displacement thereof cannot be properly set in view of the driving comfort of the vehicle and the tubular portion cannot be quickly returned to the original position under the force of the soft rubber member.
Further, if coil springs the spring constants of which can be freely set are employed in order to absorb longitudinal impacts instead of the soft rubber bushings, each connection of the suspension arm to the vehicle body inherently becomes too large in length though the stiffness of the connection can be freely set. On the other hand, when the length of the coil spring is limited in order to shorten the connection of the suspension arm to the vehicle body, it becomes very difficult to properly set the spring constant and the urging force of the coil spring, and at the same time, parts of the coil spring interfere with each other when the coil spring is compressed, thereby shortening the service life of the coil spring.